1. Field of the Invention
The present invention relates to coatings and coated thermoplastic substrates, and more particularly relates to solventless and primerless ultraviolet radiation cured silica containing coatings on thermoplastic substrates.
2. Description of Related Art
Thermoplastic substrates, such as polycarbonate sheet and film, have found many commercial applications including for example glazing or automotive headlamp applications. One widely used thermoplastic substrate is bisphenol A polycarbonate such as that known as LEXAN.RTM. polycarbonate resin, sold by General Electric Company. While polycarbonate resin is a tough transparent material, it is generally desirable to improve its abrasion resistance and weathering resistance. Prior attempts to improve the abrasion resistance of these substrates have involved heat curing silicone coating systems onto the substrates. Heat cured silicone coatings have generally required the use of primer layers to promote adhesion to the substrate. Heat cure processes are slower than ultraviolet radiation cured coating systems. Ultraviolet radiation cured abrasion resistant coating compositions have been developed which can be applied directly to a thermoplastic substrate without the use of a primer layer. For example, Chung, U.S. Pat. No. 4,486,504, discloses a solventless ultraviolet radiation-curable coating composition comprising (a) colloidal silica, (b) the acid hydrolysis product of an acryloxyfunctional or glycidoxyfunctional silane or mixtures of the functional silanes, and (c) a photoinitiator. Chung '504 discloses that the coating composition may be applied directly to a substrate, such as polyester film, and cured in one pass of UV radiation in a matter of seconds to yield adherent, abrasion resistant coatings. Another abrasion resistant UV curable coating composition is disclosed in Chung, U.S. Pat. No. 4,478,876, which teaches a composition comprising a multifunctional acrylate monomer, colloidal silica, acryloxyfunctional silanes and photoinitiators. Furthermore, Olson, et. al., U.S. Pat. No. 4,455,205, discloses a UV curable silica hardcoat containing a photoinitiator, the hydrolysis product of silylacrylate aqueous colloidal silica, and optionally a multifunctional acrylate. While the above UV radiation curable silica coatings provide adherent and abrasion resistant coated articles when cured on polycarbonate substrates, for outdoor applications it is desirable to improve upon their weatherability.
Accordingly, one object of the present invention is to provide a UV curable silica coating composition providing cured coatings exhibiting improved weatherability.